Envy
by RegenesisX
Summary: In which Yukio just can't get used to his brother's new boyfriend.


**Author's Note:** I didn't mean to write this, it just kind of... slipped out. It's also probably really messy, since I haven't tried to write anything coherent in a long time.

* * *

 **Envy**

Yukio covertly watched his twin flip agitatedly through his textbook from behind the safety of his hardcover novel. It was no secret that Rin had the worst study habits out of everyone in his class. Yukio had assigned the report his brother was working on over a week ago, and Rin had only just began to work on it. Not to mention the fact that he was always sleeping through lectures, so he never took any notes.

Yukio sighed.

Despite Rin's claims, Yukio honestly didn't like seeing his brother struggle. He admired Rin's iron will and ability to blast his way through situations, however bleak things looked. School, however, was one adversary Rin had yet to defeat.

Yukio was good at school. And he couldn't help lording that over his brother, even if it was only in his mind. He liked playing the superior one. It gave him a sense of security.

"Hey, Rin?"

The scrawling of Rin's pencil didn't miss a beat. "Yeah?"

"If you want, I could—"

A tapping noise sounded from the window, and Yukio's hands reflexively tightened around the edges of his book.

Without looking up, Rin unlatched the window. Amaimon, their half-brother, crawled in, careful to avoid messing up the work scattered across Rin's desk.

"Did you get kicked out again?"

"I was planning to go out, but it's uncomfortably hot." Amaimon paused, glancing around the room. "...Actually, I think it's hotter in here."

Rin finally lifted his head. "Is it? I haven't even noticed."

"Ah? You're capable of concentrating that hard?"

"Tch. Shut up before I hit you."

"Oooh. Scary."

Amaimon discarded his coat and vest, tossing them carelessly on the floor. As he loosened his tie, he leaned over to see what Rin was working on.

"Is that for your class?"

"Yeah... I lost the assignment and only just found it again yesterday when I was trying to clean," Rin said. He scooted forward in his chair without being asked so Amaimon could squash behind him. "It's due Friday, of course."

"I'll help you, if it'll make you get done faster."

Rin's expression brightened and he twisted around to grin at Amaimon. "Really? You're the best."

Looping his arms around Rin's waist, Amaimon nuzzled into his neck. "I want to play with you."

"This first, then that. Okay, so I have to—Oh!" Rin turned toward Yukio. "What did you want to ask me?"

Snapping his book shut, Yukio sat up and slipped on his shoes. "I'm going to head to the store. Do you want me to get you anything?"

"Uhh... Ah! Some popsicles would be nice. Thanks, Yukio!"

Yukio left the room, trying hard to curb his frustration. He never minded the idea of Rin getting into a relationship; it was a positive thing for him to make friends at all. But a demon king—Rin's _half-brother—_ wasn't anywhere near what Yukio had in mind for that sort of thing.

Rin and Amaimon started out trying to kill each other, which eventually simmered down to an uneasily picking on one another (or, Amaimon picking on Rin) until the negative energy dissipated altogether. Outside of class, they were hardly ever not together.

 _"_ _Yukio... You're probably gonna think it's weird, but this guy and I are gonna try dating. So, be nice to him, okay?"_

Yukio didn't have a problem with Rin dating a guy. But not only was Amaimon a _demon king_ , but he also infringed on something very precious to Yukio: Brother Territory.

With Amaimon always there, Rin didn't _need_ anything in particular from Yukio. Amaimon knew more about exorcism than exorcists and he was one of the most powerful demons in Gehenna. Anything _cool_ about Yukio, Amaimon could easily one-up. He'd been demoted to "errand boy."

He did his best not to show his anger, and Rin seemed none the wiser, but he was fairly certain Amaimon had picked up on it. On the rare occasions Yukio caught his eye, Amaimon's expression barely changed, but the silent challenge was there. Like he was saying, _if you want him back, try to take him from me._

As the King of Earth was the sort of person who didn't count fights as over unless he'd won, Yukio wasn't interested in getting into a confrontation, so for Rin's sake, he set his personal feelings aside.

Mostly.

* * *

More than his visits during the day, Yukio hated when Amaimon decided to spend the night. He was unhappy with suddenly acquiring a third roommate. It was an invasion of his privacy. Not that Amaimon was particularly invasive—he kept out of Yukio's side of the room, at least—but Yukio never got a good night's rest when he was there. He tossed and turned until he fell asleep, and woke up surly in the morning. Rin had even commented on his grouchiness, and if _Rin_ noticed, then Amaimon definitely had, too.

But the sight of _his_ twin wrapped up in the arms of a full-blooded demon made Yukio's blood boil. Sometimes he left the room without waking either of them up if he was in too foul of a mood. What he hated even more was seeing that Ukobach had prepared three plates for breakfast instead of two.

"It's not like he _lives_ here," Yukio grumbled, slamming down his plate just a little bit too hard on the table.

Rin bolted in a few minutes later, obviously having gotten dressed in a hurry. The buttons on his shirt weren't done up correctly and his tail waved about erratically. Amaimon strolled in after him, still wearing the t-shirt and sweatpants Rin had loaned him.

"Yukio! Why didn't you wake me up?" Rin pitched himself into a chair, shoveling food into his mouth.

"I've told you to set an alarm, haven't I?"

"Tch... I don't always wake up to it."

"Then you'll just have to figure out another way."

Rin shot him a glare. "It's easier if you wake me up."

"That's impractical. We won't be living together forever."

"Just wake me up until I find something that works, okay?"

Yukio opened his mouth to refuse, but clamped it shut again when Amaimon took a seat next to Rin. He scooted over until their shoulders brushed, making Yukio's stomach roll.

"You need help getting up in the mornings?" he asked.

Rin at least had the decency to look apprehensive. "Well, I mean, I don't wanna keep being late to class..."

"I don't mind waking you up. Any excuse to play with you is alright with me."

"Just... don't knock the dorm down."

Amaimon slid an arm around Rin's shoulders, speaking softly into his ear. "I'll be very careful."

Out of the corner of his eye, Yukio caught Rin's cheeks turning red when Amaimon scraped his teeth against his ear.

"Your shirt isn't buttoned properly."

"Huh? Oh, shit..."

Yukio stood up and deposited his half-finished breakfast on the counter. He wasn't hungry anymore.

* * *

Yukio had only walked in on his brother having sex once.

Before then, he'd managed to avoid witnessing enough of their physical relationship to the point where he could pretend it wasn't happening. Of course, just to make sure he didn't ruin that illusion for himself, Yukio was careful about where he went when Rin and Amaimon disappeared together. It was only on a day when he'd come home from a particularly tiring mission that he'd forgotten to knock on their bedroom door before carelessly flinging it open.

He was less mortified than he thought he'd be, which probably mortified him more than seeing his twin in the throes of passion underneath a demon. The crisp detail with which he remembered the half-lidded, slack-jawed expression on Rin's face the second before horror of being caught in the act set in was unsettling. He wanted to forget that face—the face he wouldn't have been able to imagine before really _seeing_ it...

Rin came downstairs an hour or so later to apologize. "Yukio, I, uh..."

"It's alright. Sorry I didn't knock."

"I just wasn't expecting you until later."

"I decided to wait on my report to Mephisto until morning," Yukio said, looking out the window. The setting sun had dipped its toes beneath the horizon, casting a golden glow over True Cross Academy. "That's why I'm so early."

"Sorry. I should've—S-Sorry. I'll be more careful next time."

That _next time_ hung in the air like a foul odor. Or perhaps it was just Yukio who felt that way.

"Perhaps it's not _me_ you should be careful around," slipped out of his mouth.

"...What do you mean?"

"Is it really wise for an exorcist to be sleeping with a demon?"

"I wouldn't tell an exorcist to do it." Rin suddenly slammed his hands down on the table, forcing Yukio to look up. His twin's head was bowed, shoulders trembling with an unsteady exhale. "I'm a demon, Yukio. Things would be different if I _wasn't like this._ "

Yukio hadn't meant it like that. He didn't know how he'd meant it.

No, he'd wanted it to hurt.

Now he was regretting it.

"Rin, I—"

"...If anyone else finds out that I'm the son of Satan, they won't look at me the same way anymore. I'll be less _human_. But I'll always be more human than Amaimon." Rin peeked out at him through the fringe of his hair. "For the first time, I'm not worried about hurting someone, Yukio."

Yukio hadn't walked in on them after that. Either Rin was taking more precautions, or Yukio was avoiding them more, or both. Still, every once and a while he saw evidence of their activities.

When Rin returned from their break between classes, Yukio's eyes immediately zeroed in on a set of bruises on his twin's neck. Yukio caught his sleeve as he passed by, pulling him aside.

"Must you be so indecent at school?"

Rin's eyes widened and he slapped a hand to cover his neck. "That bastard, I told him not to leave marks...!"

"You really expect him to listen to that?" Yukio straightened Rin's shirt, buttoning it all the way up. "You'll just have to cover them until they go away. Unless you want Shima pestering you about your _girlfriend_ again."

"Yeah, yeah..."

"...If you keep letting this happen at school, I'll be forced to do something about it, Rin."

That was mostly a lie. Their activities had yet to cause a disruption in class, so he didn't _have_ to do anything. It was just _him_ that was bothered by it.

Rin's expression hardened slightly and he stepped back, making the collar of his shirt sliding out of Yukio's hands. "I'll talk to him about it."

* * *

There was a small abandoned area near the old dorm that Rin and Amaimon used to spar in after cram school. Yukio always went with them to supervise, even though there was nothing he could actually do if either of them got out of control. According to Rin, Amaimon was "teaching him" how to use his demonic powers properly. Yukio was fairly certain Amaimon just wanted an excuse to _play_ with Rin.

Yukio had expressly forbade them from going all-out on each other, and thus far they'd respected that rule, which, he was reluctant to admit, probably had something to do with the fact that they were dating now.

Rin's sword sat beside Yukio on the rotting bench, unsheathed. Its flames flickered quietly, not even seeming to consider burning through the wood it was sitting on. Rin and Amaimon were working nearby for once, seeming to be concentrating on close-range combat. Rin had very little formal training when it came to hand-to-hand because he would've easily laid-out any human opponents with his demonic strength, but his lack of finesse was obvious against Amaimon. He tended to throw all of his power into one move, which Amaimon usually managed to dodge or turn the attack back against him.

Amaimon nimbly ducked under Rin's incoming fist and landed a solid kick on his stomach without so much as batting an eye. Rin was sent flying backward, hitting the ground with a thud. Groaning, he rolled onto his side.

"You waste too much energy on your attacks without being certain whether they'll hit or not."

"Tch... If there's still a chance, it's worth it."

"It's easier if you just learn to fight better." Crouching down, Amaimon offered Rin a hand to help him back up. "You act like your left hand doesn't know what your right hand is doing. Be more aware of how you hold your body. Otherwise, you're too clumsy."

Amaimon wasn't a bad teacher when he was motivated. Plus, he had his own set of demonic powers, so he could explain practical ways for Rin to bend the blue flames to his will. Yukio would never have been able to do that. Amaimon also had the same sort of energy level that Rin did—not being finished until you were too tired to move.

Yukio didn't possess the strength Amaimon did. The King of Earth could crack rocks and split trees and shake the ground. His opposition crumbled to dust underneath his feet. Yukio had seen him with his shirt off a handful of times, and he was entirely muscle. As a mere human, Yukio paled in comparison.

It sucked.

* * *

Yukio was sitting at the fountain outside of True Cross Academy's main building, eating his lunch alone, when the small white dog that was Mephisto Pheles took a seat next to him. Actually, he didn't even noticed Mephisto then, until the principal leaned over and snatched a bit of chicken out of his bento box.

"M-Mephisto!" he squeaked in a voice unlike his usual self. "What are you doing here?"

"You've seemed depressed, so I came to see what was the matter. You can come to me if you're having problems. I'm technically your guardian, after all."

"It's..." Yukio stared down at his food. "It's not really my problem. Or, rather... I shouldn't get myself so involved."

"With what?"

"Rin's... gotten into a relationship." He didn't go into the specific details, because sabotaging his twin's relationship by exposing that his boyfriend was a demon was pretty low, even for him. "And I don't care for the guy he's with."

"You think he's bad for you brother?"

Yukio sighed, pulling off his glasses to clean the lenses. "Not... exactly. It's almost like he's _too_ good for Rin."

Mephisto made a funny choking noise, which Yukio ignored.

"He's obnoxious and petty and childish, but he's something that no one else can be to Rin. Not even... Not even me."

"Are you jealous?"

"...Yeah." A wry laugh bubbled up in his throat, despite the tears that threatened his already-blurry vision. "It's stupid, but I liked being the _coolest_ to him. But now he's got someone else to look up to, and he doesn't... he doesn't need _me_ anymore."

"Oh, I wouldn't be too sure about that. Rin is pretty stubborn when it comes to you, Yukio. I would imagine that his boyfriend has similar complaints about you."

"...What do you mean?"

"I mean you aren't as easily replaceable in Rin's mind as you think. If you ask this... _boyfriend_ , I'm sure he'll tell you how annoying it is to hear about amazing little brother Yukio all the time."

"...I'm not that amazing."

"That isn't my judgment call to make." Mephisto hopped down from the fountain and trotted off. "If you want to know the truth, seek it out."

* * *

Very seldom was Amaimon without Rin inside the dorm, but it wasn't impossible for Yukio to pass him alone in a hallway or on the stairs. Usually, Yukio averted his eyes and quickened his pace, but today he stopped.

"...Amaimon."

"Hm?"

Yukio turned to face him. Amaimon, who had stopped several stairs down, leaned up against the railing and lazily looked back up at him.

"I want you to break up with my brother."

"I thought you'd eventually say something like that to me. Took you longer than I expected. But I can't do that."

"Why can't you?"

Amaimon slouched against the railing, stretching one of his long arms out across the banister. "He belongs to me."

"Give him back... Give him back, you son of a bitch!"

Yukio hadn't taken into account the possibility of rage taking control of his body, but even if his actions had been premeditated, he wouldn't have attacked Amaimon on a stairway. As it was, the King of Earth easily deflected his right hook and sent him tumbling down the remainder of the steps into the stairwell.

Hissing in pain, Yukio rolled onto his back and found himself staring straight up at Amaimon.

"You never had him in the first place. Although, I'd really love to get rid of you."

Amaimon's foot came down on him hard, the heel of his boot digging sharply into his stomach.

"You're like this ugly stain that won't come out of a rug. No matter how close he and I get, Rin is always more worried about you and it pisses me off. It would be easier if I just killed you."

"Tch... Why don't you, then?"

"Because Rin is unreasonable when it comes to you. He would hate me if I killed you, regardless of my motivation. Normally, that wouldn't bother me, but..." Amaimon removed his foot. "Rin has made me unreasonable, too."

Yukio stared up at him, speechless.

"Don't assume that just because I'm a demon, I can't value him."

Yukio wasn't sure he'd ever questioned Amaimon's sincerity with regards to Rin. "...If you didn't really like him, you would've gotten bored and left already."

Amaimon tilted his head. "Well. That's true."

Heaving himself up, Yukio let out a tense breath and stared down at the floor. "I guess it's all really just in my head."

"Don't get me wrong... I don't like you. But you're my little brother, too, so..."

Amaimon pulled something out of his pocket and thrust it under Yukio's nose. It was one of the suckers the demon constantly ate. Taking it from him, Yukio glanced up at Amaimon in surprise.

"Don't act like we've excluded you when you haven't even asked to be part of the conversation."

With that, Amaimon walked past Yukio and hopped down the remainder of the stairs two at a time until he reached the ground floor, singing some jingle from a TV commercial as he skipped off. Yukio stared down at the candy in his hand and shook his head, a chuckle bubbling up in his chest.

"I feel... lighter. Damn you, Amaimon."

* * *

 _Shocked_ didn't even begin to cover how Rin felt when he came downstairs to find Yukio casually eating breakfast with _Amaimon_ , of all people.

Stopping short in the doorway, he scrubbed his eyes to make sure all the sleep was out of them before looking again. But sure enough, there was Yukio sitting across from his boyfriend, slowly turning a page of his book while he bit into an apple. Amaimon had his arms folded on the table, resting his head on them—something he tended to do when he was letting his mind wander. Yukio finally noticed Rin when he had to set his apple down to take a sip of coffee.

"Oh. Good morning, Rin."

"G-Good morning...?" Rin slowly approached the two, fearing any sudden movement might cause the scene to erupt into chaos. "What are you doing?"

Yukio furrowed his brow. "Eating breakfast. What does it look like?"

Lifting his head, Amaimon stretched his arms up to tug Rin down for a lazy kiss. "You were slow."

"I was sleeping—N-No! I mean, what are you two doing... together?"

"Are we not allowed to be?" Yukio asked.

"W-Well, no, but I just thought... Y'know." Rin shrugged, awkwardly rubbing the back of his neck. "I thought you two hated each other."

"We do." Amaimon tilted his chair back, bracing his knees against the edge of the table. "Yukio is boring and he tries to act superior, even though he's the youngest."

"It isn't my fault you're irresponsible. Please get your legs off the table."

Amaimon let the front legs of his chair drop back to the floor. "Save me, Rin."

"But you... Since when did you two even speak to each other?" Rin asked. "I don't think either of you have said three words to each other."

Yukio closed his book. "You're serious about him, aren't you, Rin?"

"Y-Yeah... Of course I am."

"Then I'll just have to get used to him being here. Which shouldn't be too difficult." Yukio shot him a smirk. "I've gotten used to you, after all."

Rin pouted. "What the hell is that supposed to mean?"

"It means the two of you are more alike than you think." Rising from his chair, Yukio picked up his plate. "I have a meeting to attend. I know it's your day off, but try not to waste all of your time fooling around."

"Tch. You really are boring, moley four-eyes. But I'm happy... I," Rin's voice quavered a little, "thought you were starting to hate me."

Genuine surprise flashed across Yukio's face. "Rin... I could never hate you. I'll see you later, okay?"

"Y-Yeah..." Rin flopped down in Amaimon's lap, watching his younger brother leave the cafeteria. When Yukio disappeared from view, he turned to Amaimon. "You said something to him."

"I just... clarified myself, I guess." Amaimon wrapped his arms around Rin's waist, resting his forehead against the back of his neck. "He couldn't figure out the difference between what I am to you, and what he is to you, so he thought I was trying to take his place."

Rin frowned, staring at Yukio's empty chair across from him. "That should be obvious though, right? I mean, he knows what being _in a relationship_ means, right?"

"I'm not in a position to say whether or not his feelings are more than just familial."

Swallowing hard, Rin reached back to tangle a hand in Amaimon's hair. "Y-You don't think Yukio feels... _those_ things toward me, does he?"

"I couldn't say. But it doesn't matter, because you belong to me."

Rin immediately felt his body grow hot at Amaimon's straightforward proclamation, followed by a pair of lips on his neck. Things like _I love you_ hadn't come into their conversations yet, but Rin wouldn't hesitate to say it. It was Amaimon who preferred to communicate his feelings wordlessly, and Rin never had trouble understanding.

Yukio, however...

"Ah... I've probably been brushing him off, haven't I," he said. "I'll have t' do something nice for him later, like clean the bathroom..."

"Mm, me first, though." Amaimon's hands slid lower, pawing at him through his clothes, making Rin gasp. "I'm in the mood to thoroughly mark you up."

Jealousy was an ugly creature, and Rin felt guilty for enjoying it at his younger brother's expense, but not quite enough to make him stop Amaimon from dragging him upstairs to defile him on Yukio's bed.

* * *

 **A/N:** And then they had mad hot buttsex

Rin/Amaimon is my new otp

 _ **RegenesisX**_


End file.
